E-mail distribution control method and mail server

ABSTRACT

When gateway server  30  receives an email, which is addressed to mobile station  10 , gateway server  30  transmits an email arrival notice to mobile station  10 . When gateway server  30  receives an acknowledgement response (referred to as ‘Ack’ hereinafter) responding to the email arrival notice from mobile packet communication network  20 , gateway server  30  evaluates the status of mobile station  10  on the basis of the type of Ack, and determines whether to transmit the email or not. Moreover, gateway server  30  modifies the time interval for retransmitting the email arrival notice to mobile station  10  according to the status of mobile station  10  when the transmission of the email arrival notice fails.

TECHNICAL FIELD

[0001] The present invention relates to a method of controlling emaildelivery and an email server for retransmitting emails to mobilestations, such as mobile phones, in a mobile communication network.

BACKGROUND ART

[0002] In recent years, several kinds of mobile communication systemsproviding email services to users of mobile stations, such as mobilephones, which are connected to the Internet through various mobilecommunication networks, have come into use. In the mobile communicationsystems, emails addressed to subscribers of mobile stations are at firstreceived by email servers, who in turn deliver the emails to thesubscribers of mobile stations.

[0003] Generally speaking, in a mobile communication network, thecommunication status of a mobile station is unstable, and can causecommunication failures. Therefore, when emails are delivered during acommunication failure, a mobile station is unable to carryout theroutine function of receiving the emails, which would produce anincrease of communication traffic from the large number of undeliveredmail leading to a traffic congestion, especially on narrowband networkssuch as mobile packet communication networks.

DISCLOSURE OF THE INVENTION

[0004] The present invention has been made in the light of the foregoingdescription of the prior art, and it is an object of the presentinvention to provide a method of controlling email delivery and an emailserver, which can deliver emails through a mobile communication networkby maintaining a smooth flow of email traffic even when there is anincrease in the number of deliveries; and, can prevent trafficcongestion of email deliveries while retransmitting emails, whoseearlier attempts at delivering to mobile stations failed.

[0005] To solve the above-mentioned problem of the prior art, thepresent invention proposes a method of controlling email delivery, whichcomprises: a receiving step for receiving an email, which is addressedto a mobile station; a status obtaining step for obtaining informationon communication status of the mobile station from a mobilecommunication network to which the mobile station belongs; atransmission determining step for determining whether the email shouldbe transmitted to the mobile station at the present moment or not, onthe basis of the information on the communication status; a transmittingstep for transmitting the email if the email is determined to betransmitted in the transmission determining step, and; a retrialinstructing step for setting a waiting time on the basis of theinformation on the communication status, and instructing the executionof obtaining operation by the status obtaining step following thewaiting time, if the email is determined to not be transmitted in thetransmission determining step, or if the email is determined to betransmitted in the transmission determining step and transmission of theemail fails. This is the first mode of the proposed method ofcontrolling email delivery of the present invention.

[0006] Another preferred mode of the method of controlling emaildelivery of the present invention comprises all characteristics of thefirst mode of the method of controlling email delivery; and furthercomprises an arrival notice transmitting step for transmitting a noticeof arrival of the email to the mobile station, when the email, which isaddressed to the mobile station, is received, and; information oncommunication status with regard to reception of the notice of arrivalis obtained as the information of communication status in the statusobtaining step. This is the second mode of the proposed method ofcontrolling email delivery of the present invention.

[0007] Another preferred mode of the method of controlling emaildelivery of the present invention comprises all characteristics of thefirst mode of the method of controlling email delivery; and theinformation on the communication status, which is obtained in the statusobtaining step, indicates any of the following: communication possiblestatus, communication busy status, or communication impossible status;in the retrial instructing step, a first time period is set as thewaiting time if the information on the communication status indicatescommunication busy status, and a second time period, which is longerthan the first time period, is set as the waiting time if theinformation on the communication status indicates communicationimpossible status. This is the third mode of the proposed method ofcontrolling email delivery of the present invention.

[0008] Another preferred mode of the method of controlling emaildelivery of the present invention comprises all characteristics of thefirst mode of the method of controlling email delivery; and the waitingtime is determined on the basis of number of times of execution of thestatus obtaining step for the email up to the present moment. This isthe fourth mode of the proposed method of controlling email delivery ofthe present invention.

[0009] Another preferred mode of the method of controlling emaildelivery of the present invention comprises all characteristics of thefourth mode of the method of controlling email delivery; and in theretrial instructing step, if the number of times of execution of thestatus obtaining step for the email up to the present moment is the sameas or more than a predetermined number, a predetermined time period isset as the waiting time. This is the fifth mode of the proposed methodof controlling email delivery of the present invention.

[0010] Another preferred mode of the method of controlling emaildelivery of the present invention comprises all characteristics of thefirst mode of the method of controlling email delivery, and theinformation on the communication status is information of strength ofreceived radio wave of the mobile station. This is the sixth mode of theproposed method of controlling email delivery of the present invention.

[0011] Another preferred mode of the method of controlling emaildelivery of the present invention comprises all characteristics of thefirst mode of the method of controlling email delivery, and theinformation on the communication status is information of transition ofstrength of received radio wave of the mobile station. This is theseventh mode of the proposed method of controlling email delivery of thepresent invention.

[0012] Moreover, an email server of the present invention comprises: areceiving means for receiving an email, which is addressed to a mobilestation; a status obtaining means for obtaining information oncommunication status of the mobile station from a mobile communicationnetwork, to which the mobile station belongs; a transmission determiningmeans for determining whether the email should be transmitted to themobile station at the present moment or not on the basis of theinformation on the communication status; a transmitting means fortransmitting the email if the email is determined to be transmitted bythe transmission determining means, and; a retrial instructing means forsetting a waiting time on the basis of the information on thecommunication status, and instructing the operation of the statusobtaining means following the waiting time, if the email is determinednot to be transmitted by the transmission determining means.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a block diagram showing a configuration of a mobilecommunication system according to the first embodiment of the presentinvention.

[0014]FIG. 2 is a block diagram showing a configuration of a mobilestation according to the first embodiment of the present invention.

[0015]FIG. 3 is a block diagram showing a configuration of a gatewayserver according to the first embodiment of the present invention.

[0016]FIG. 4 is a format chart showing a mailbox administration table,which is stored in a memory of an email delivery-managing unit,according to the first embodiment of the present invention.

[0017]FIG. 5 is a graph showing a relation between values of counters,which an email delivery-managing unit sets, and time intervals forretransmissions of arrival notices.

[0018]FIG. 6 is a flowchart showing a process, which is executed by anemail delivery-managing unit according to the first embodiment of thepresent invention.

[0019]FIG. 7 is a block diagram showing a configuration of a mobilestation according to modification (1) of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0020] In the following paragraphs, an embodiment of the presentinvention is explained with reference to drawings. The embodimentexplained below, is a system where the present invention is applied to amobile communication system, which is connected to the Internet.

[0021] [1] Embodiment

[0022] [1.1] Configuration of Embodiment

[0023] (1) Configuration of System

[0024]FIG. 1 is a diagram showing a configuration of a mobilecommunication system in this embodiment. The mobile communication systemin this embodiment contains a plurality of mobile stations 10A, 10B . .. , mobile packet communication network 20, base station 21, servicecontrol station 22, gateway server 30, Internet 40, and a plurality ofterminals 50A, 50B . . . , etc. In FIG. 1, only mobile stations 10A and10B, and terminals 50A and 50B, are shown representing a plurality ofmobile stations 10A, 10B . . . in the mobile communication network, anda plurality of terminals 50A, 50B . . . in the mobile communicationnetwork, respectively, to simplify the diagram.

[0025] Terminals 50A, 50B . . . (referred to as ‘terminals 50’hereinafter, if there is no need to distinguish them from each other),are terminals, such as personal computers, which are connected toInternet 40. Terminal 50 has a keyboard (not shown) and a computer mouse(not shown) by which its user inputs data; a display unit (not shown)for displaying text messages and images; as well as, several componentsfor data communication with other communication devices through Internet40.

[0026] Moreover, a program for composing, transmitting, and receivingemails is installed in terminal 50, and terminal 50 functions as anemail client. In this description, ‘an email client’ means an apparatus,which transmits and receives emails using given email addresses.Terminal 50 composes emails following particular operations carried outby its user, and executes email-transmitting processes of the emails,according to the installed program. On the other hand, terminal 50obtains information on emails addressed to terminal 50, which exist inInternet 40, automatically or following the operations carried out byits user, and displays the information in the above-mentioned displayunit.

[0027] Mobile stations 10A, 10B . . . (referred to as ‘mobile stations10’ hereinafter, if there is no need to distinguish them from eachother), are mobile communication terminals, such as mobile phones andPersonal Handy-phone Systems (PHSs; registered trademark), which can usea communication service in mobile packet communication network 20, andexecute data communication and voice communication through mobile packetcommunication network 20. Each of mobile stations 10 has an emailaddress for transmitting and receiving emails and it can function as anemail client.

[0028] Mobile packet communication network 20 is a communicationnetwork, which provides packet communication service for mobile stations10. Mobile packet communication network 20 has gateway server 30, aplurality of base stations 21, and service control station 22, as wellas, a packet subscriber management apparatus (not shown). Base station21 executes data communication with mobile station 10 through a radiocommunication device (not shown). Service control station 22 has aposition registration table. In the position registration table, data oflocations of mobile stations 10, namely data of service areas wheremobile stations 10 are located, are stored with IDs of mobile stations10 corresponding to the data of the service areas. In this description,an ID of mobile station 10 means an identifier, which provides exclusiveidentity to each mobile station 10 in mobile packet communicationnetwork 20. Contents of the position registration table are updated by aposition registration operation, which is executed by mobile station 10,and the position registration operation is the same as the one in aconventional system. Hence, an explanation of the operation is omittedin this description.

[0029] The position registration table is referred to obtain a locationof mobile station 10 and call mobile station 10 when there is an emailarrival for mobile station 10. To be more specific, when mobile packetcommunication network 20 receives an email addressed to mobile station10, mobile packet communication network 20 identifies the service areawhere mobile station 10 is located on the basis of the positionregistration table of service control station 22. Then, mobile packetcommunication network 20 transmits to mobile stations 10, an emailarrival notice from all base stations 21 in the service area. Whenmobile station 10 receives the email arrival notice, it transmits anacknowledgement response with regard to the email arrival notice. Inthis embodiment, there are two kinds of acknowledgement responses, whichmobile station 10 transmits in response to email arrival notices, asdescribed below:

[0030] a. Normal acknowledgement response (referred to as ‘normal Ack’hereinafter)

[0031] This response is an acknowledgement response, which indicatesthat mobile station 10 is now available.

[0032] b. Busy acknowledgement response (referred to as ‘busy Ack’hereinafter)

[0033] This response is an acknowledgement response, which indicatesthat the communication line of mobile station 10 is now busy.

[0034] In the instance that no acknowledgement response is transmittedfrom mobile station 10 within a predetermined time period after an emailarrival notice is transmitted to mobile station 10, mobile packetcommunication network 20 transmits a negative acknowledgement response(referred to as ‘NAck’ hereinafter) to the node of the transmittingside, which indicates that mobile station 10 is not available now. Onthe other hand, in the instance that an acknowledgement response to anemail arrival notice is transmitted from mobile station 10 within apredetermined time period after the email arrival notice is transmittedto mobile station 10, mobile packet communication network 20 forwardsthe acknowledgement response to the node of the transmitting side. Inthe following explanation, ‘Ack’ is used as a general term for normalAck, busy Ack, and NAck, when there is no need to distinguish them fromeach other. This is a summary of the calling operation carried out by amobile station using the position registration table.

[0035] Gateway server 30 is a computer system provided to a packetgateway switching and relaying station, which interconnects Internet 40and mobile packet communication network 20. Gateway server 30 functionsas an email server, which relays email communication between emailclients (namely, mobile stations 10 and terminals 50). To be morespecific, gateway server 30 receives emails, which should be transmittedfrom terminals 50 in the side of Internet 40 or mobile stations 10 inthe side of mobile packet communication network 20 to other mobilestations 10. Then, gateway server 30 transmits email arrival notices andthe received emails to mobile stations 10 to which the emails areaddressed.

[0036] If gateway server 30 transmits an email to mobile station 10 andmobile station 10 receives the email successfully, mobile station 10transmits a reception success notice to gateway server 30 through basestation 21. On the other hand, if mobile station 10 fails to receive theemail, mobile station 10 does not transmit a reception success notice.In such a case, base station 21 confirms that no reception successnotice was transmitted in a predetermined time period, and transmits areception failure notice to gateway server 30.

[0037] Gateway server 30 receives reception success notices, receptionfailure notices, and Ack as mentioned above, and judges whether theemail arrival notice needs to be retransmitted on the basis of theinformation given by these notices. At the same time, gateway server 30evaluates the communication status of mobile station 10 on the basis ofthe information, and sets the proper retransmission times of the emailarrival notices. The time setting operation is precisely explained inthe latter part of the following description.

[0038] (2) Configuration of Mobile Station 10

[0039]FIG. 2 is a block diagram showing a configuration of mobilestation 10 in this embodiment. Mobile station 10 has control unit 11;transmitting and receiving unit 12; user interface unit 13 (referred toas ‘user I/F unit 13’ hereinafter); and bus 14, which interconnectsthese units.

[0040] Transmitting and receiving unit 12 executes radio communicationbetween mobile packet communication network 20 and base station 21. UserI/F unit 13 has a liquid crystal display unit (not shown), fordisplaying dialog screens and various kinds of information; a key pad(not shown) by which the user inputs various kinds of data; a microphone(not shown); and a speaker (not shown) by which the user talks withother people, etc.

[0041] Control unit 11 comprises CPU 111, ROM 112, and RAM 113, and itcontrols each component of mobile station 10. ROM 112 stores mobilestation IDs for identifying each mobile station 10 in mobile packetcommunication network 20; control programs and various kinds of controlinformation for controlling the whole of mobile station 10; a programfor establishing communication connection between gateway server 30 andmobile station 10; and a program for composing, transmitting, andreceiving emails. RAM 113 is used as the work area of CPU 111; and CPU111 executes various kinds of processes by reading out control programsfrom ROM 112 to ROM 113 and following the programs. If mobile station 10receives an email, CPU 111 writes the email in RAM 113. And when theuser instructs mobile station 10 to display the email using the key pad(not shown), etc. of I/F unit 13, for example, CPU 111 displays themessage of the email in the liquid crystal display unit (not shown) ofI/F unit 13.

[0042] (3) Configuration of Gateway Server 30

[0043]FIG. 3 is a diagram showing a configuration of gateway server 30in this embodiment. Gateway server 30 has system control unit 31,subscriber information administration unit 32, email delivery-managingunit 33, and mailbox 34.

[0044] System control unit 31 controls each component of gateway server30. At the same time, system control unit 31 functions as an interfacebetween different communication networks, and executes protocolconversions between mobile packet communication network 20 and othercommunication networks such as Internet 40. For example, if systemcontrol unit 31 receives data addressed to terminal 50 from mobilestation 10, control unit 31 converts the format of the data from theprotocol used in mobile packet communication network 20 into theprotocol used in Internet 40, and transmits the data to Internet 40.

[0045] Subscriber information administration unit 32 stores andadministrates information concerning subscribers of the packetcommunication service provided by mobile packet communication network20. The information contains telephone numbers and email addresses ofmobile stations 10A, 10B . . . , attributive information of thesubscribers, and so on.

[0046] Email delivery-managing unit 33 manages delivery of emailsbetween email clients. To be more specific, email delivery-managing unit33 receives emails addressed to mobile station 10, and stores them inmailbox 34 temporarily. Then, email delivery-managing unit 33 transmitsthe stored email to mobile station 10, to which the email is addressed.In mailbox 34, each mobile station 10 is allotted a memory addressaccording to its email address, and email delivery-managing unit 33stores the email at the memory address which corresponds to the emailaddress of mobile station 10, to which the email is addressed.

[0047] As mentioned above, when an email arrival notice needs to beretransmitted to mobile station 10, gateway server 30 sets theretransmission time of email arrival notice according to thecommunication status of mobile station 10. To execute this operation,email delivery-managing unit 33 has memory 331, in which a mailboxadministration table is stored as shown in FIG. 4. The mailboxadministration table stores mobile station IDs, names of emails storedin mailbox 34, timing information, and counters, which are arrangedaccording to email addresses. The timing information shows the timeswhen email arrival notices should be retransmitted to each mobilestation 10, and email delivery-managing unit 33 retransmits emailarrival notices following the timing information. The counters areincreased by ‘1’ when email delivery-managing unit 33 does not receivean Ack within a certain time period after an email arrival notice istransmitted, or when email delivery-managing unit 33 receives a NAck.

[0048] [1.2] Operation of Embodiment

[0049] Following is a description of the operations of theabove-mentioned embodiment, whereby; an email is transmitted fromterminal 50A in Internet 40 to mobile station 10A in mobile packetcommunication network 20.

[0050] When gateway server 30 receives an email addressed to mobilestation 10A from terminal 50A, system control unit 31 forwards the emailto email delivery-managing unit 33, and email delivery-managing unit 33stores the email in mailbox 34. At the time of storing the email, emaildelivery-managing unit 33 stores the name of the email in the mailboxadministration table of memory 331 in correspondence with the emailaddress of mobile station 10A.

[0051] After gateway server 30 stores the received email in mailbox 34and stores the name of the email in the email administration table,email delivery-managing unit 33 executes an email delivery process,which is shown in FIG. 6.

[0052] First, email delivery-managing unit 33 reads out the timinginformation corresponding to the email in the mailbox administrationtable, and if the time indicated by the timing information is prior tothe current time, email delivery-managing unit 33 transmits an emailarrival notice to mobile station 10A, to which the email is addressed,through system control unit 31, according to the information in themailbox administration table (step Sa1).

[0053] Next, email delivery-managing unit 33 monitors, for a certainperiod of time, the reception of an Ack, which is a response to theemail arrival notice, and judges if the Ack is received (step Sa2).Namely, email delivery-managing unit 33 checks if gateway server 30receives an Ack from mobile packet communication network 20, repeatedly,at short regular intervals, until the Ack is received or until a certainpredetermined time period set for checking the receipt of the Ackpasses.

[0054] (1) In the Instance that no Ack is Received or a NAck is Received

[0055] In the instance that gateway server 30 does not receive an Ackwithin a certain time period, email delivery-managing unit 33 obtains a‘No’ as the result of the judgment carried out in step Sa2, and theoperation moves to step Sa10.

[0056] On the other hand, in the instance that gateway server 30receives a NAck, email delivery-managing unit 33 obtains a ‘Yes’ as theresult of the judgment of step Sa2. Then, email delivery-managing unit33 judges whether the received Ack is a NAck in step Sa3, and it obtainsa ‘Yes’ as the result of the process carried out in step Sa3, and theoperation moves to step Sa10.

[0057] As explained above, if gateway server 30 does not receive an Ackor if gateway server 30 receives a NAck, the operation moves to stepSa10, and email delivery-managing unit 33 increases the countercorresponding to mobile station 10A in the mailbox administration tableby ‘1’.

[0058] Next, email delivery-managing unit 33 reads out the updatedcounter (step Sa11).

[0059] Then, email delivery-managing unit 33 determines a time intervalto be maintained between the current time and the time when an emailarrival notice should be retransmitted (referred to as ‘retrial time’hereinafter), on the basis of the updated counter. Namely, emaildelivery-managing unit 33 determines the time interval between thecurrent time and the retrial time, and updates the timing informationcorresponding to mobile station 10A in the mailbox administration tableusing the determined time interval (step Sa12). FIG. 5 illustrates agraph showing the relation between the counter, which emaildelivery-managing unit 33 uses to calculate the time interval, and thetime interval for retransmission of the email arrival notice. As shownin the graph, where the counter is smaller than a certain value, emaildelivery-managing unit 33 calculates a time interval for retransmissionof email arrival notice in direct proportion to the size of the counter;and where the counter is larger than a certain determined value, emaildelivery-managing unit 33 calculates a time interval for retransmissionof the email arrival notice so as to be within a predetermined maximumtime interval T where all time intervals converge. The maximum timeinterval T can be set at any value.

[0060] After step Sa12, the operation of email delivery-managing unit 33returns to step Sa1, and proceeds to the above-mentioned operations orthe following operations, which are carried out according to thesituation.

[0061] (2) In the Instance that a Busy Ack is Received

[0062] In the instance that gateway server 30 receives a busy Ack, emaildelivery-managing unit 33 obtains a ‘No’ as the result of the judgmentcarried out in step Sa3 for judging whether the received Ack is a NAckor not.

[0063] Then, email delivery-managing unit 33 obtains a ‘No’ as theresult of the judgment of step Sa4 for judging whether the received Ackis a normal Ack or not.

[0064] Then, email delivery-managing unit 33 updates the timinginformation corresponding to mobile station 10A in the mailboxadministration table using the predetermined time interval, and at thesame time, email delivery-managing unit 33 initializes the countercorresponding to mobile station 10A into ‘0’ (step Sa9). According tothis operation, the new timing information, namely the retrial time,corresponding to mobile station 10A becomes the time when thepredetermined time interval passes after email delivery-managing unit 33receives an Ack.

[0065] After step Sa9, the operation of email delivery-managing unit 33returns to step Sa1, and proceeds to the above-mentioned operations orthe following operations carried out according to the situation.

[0066] (3) In the Instance that a Normal Ack is Received

[0067] In the instance that gateway server 30 receives a normal Ack,email delivery-managing unit 33 obtains a ‘No’ as the result of thejudgment carried out in step Sa3 for judging whether the received Ack isa NAck or not.

[0068] Then, email delivery-managing unit 33 obtains a ‘Yes’ as theresult of the judgment carried out in step Sa4 for judging whether thereceived Ack is a normal Ack or not.

[0069] Then, email delivery-managing unit 33 reads out the name of theemail corresponding to mobile station 10A in the mailbox administrationtable, and reads out the email, which is stored in mailbox 34 using thename of the email. At the same time, email delivery-managing unit 33reads out the mobile station ID corresponding to mobile station 10A inthe mailbox administration table, and transmits the email to mobilestation 10A through system control unit 31 using the information of themobile station ID (step Sa5).

[0070] Next, email delivery-managing unit 33 judges whether the emailwas delivered successfully, namely whether a reception success noticeresponding to the email is received from mobile station 10A (step Sa6).

[0071] (i) In the Instance that Email Delivery Fails

[0072] In the instance that gateway server 30 receives a receptionfailure notice from mobile station 10A through base station 21, gatewayserver 30 obtains a ‘No’ as the result of the judgment carried out instep Sa6.

[0073] Then, email delivery-managing unit 33 updates the timinginformation corresponding to mobile station 10A in the mailboxadministration table using the predetermined time interval, and at thesame time, email delivery-managing unit 33 initializes the countercorresponding to mobile station 10A into ‘0’ (step Sa8). According tothis operation, the new timing information, namely the retrial time,corresponding to mobile station 10A becomes the time when thepredetermined time interval passes after email delivery-managing unit 33receives an Ack.

[0074] After step Sa8, the operation of email delivery-managing unit 33returns to step Sa1, and proceeds to the above-mentioned operations orthe following operations carried out according to the situation.

[0075] (ii) In the Instance that Email Delivery Succeeds

[0076] In the instance that gateway server 30 receives a receptionsuccess notice from mobile station 10A, or in the instance that gatewayserver 30 does not receive either a reception success notice or areception failure notice from mobile station 10A after a certain timeperiod, gateway server 30 obtains a ‘Yes’ as the result of the judgmentcarried out in step Sa6.

[0077] If gateway server 30 receives a reception success notice frommobile station 10A, email delivery-managing unit 33 deletes the emailfrom mailbox 34 and deletes the name of the email from the mailboxadministration table. At the same time, gateway server 30 initializesthe counter corresponding to mobile station 10A into ‘0’, and ends theseries of operations. On the other hand, if gateway server 30 does notreceive either a reception success notice or a reception failure noticefrom mobile station 10A after a certain time period, emaildelivery-managing unit 33 only initializes the counter corresponding tomobile station 10A, and ends the series of operations (step Sa7).

[0078] [1.3] Setting of Time Interval for Retransmission of EmailArrival Notice

[0079] As mentioned above, according to the email delivery controlmethod in this embodiment, gateway server 30 can set time intervals forretransmission of email arrival notice to each mobile station accordingto an Ack or a reception failure notice, which is transmitted from eachmobile station through mobile packet communication network 20.

[0080] (1) In the Instance that no Ack is Received or a NAck is Received

[0081] In the instance that gateway server 30 does not receive an Ack orgateway server 30 receives a NAck, as mentioned above, it is preferablethat such a relation between the counter and the time interval forretransmission of email arrival notice as shown in FIG. 5 should be usedfor determining the time interval because of the following reasons. Whenmobile packet communication network 20 transmits a NAck, there areseveral possible situations as follows: mobile station 10 may be movingfrom a radio wave zone of a base station to another radio wave zone ofanother base station, and the radio wave strength of mobile station 10may have decreased temporarily (under a state of ‘decrease of radio wavestrength caused by roaming’); mobile station 10 may be located outsidethe service area; the power supply to mobile station 10 has been shutoff or; mobile station 10 has broken down. If mobile station 10 islocated outside the service area, there is a strong possibility thatmobile station 10 will be able to communicate after a short period oftime. However, if mobile station 10 has broken down, there is nopossibility of mobile station 10 transmitting an Ack even though emailarrival notices are frequently transmitted to mobile station 10. And insuch a case, gateway server 30 receives a NAck repeatedly from mobilepacket communication network 20.

[0082] On the other hand, if mobile packet communication network 20 doesnot transmit any Ack, it could be due to a problem in the mobile packetcommunication network. The problem may be just temporary and soonrectified, but in some cases it may take a long time to rectify theproblem.

[0083] Generally speaking, it can be assumed that the more the number ofreception failures of Ack and receptions of Nack, after the lastsuccessful communication between gateway server 30 and mobile station10; namely, the increase in the size of the counter in the mailboxadministration table of this embodiment; the stronger the possibilitythat mobile station 10 will not be able to communicate for a long periodof time. Under such circumstances, if email arrival notices arefrequently transmitted to mobile station 10, there is a strongpossibility that the increased data traffic due to a lot ofuntransmittable data in mobile packet communication network 20 wouldcause a traffic congestion.

[0084] To avoid such a problem as mentioned above, emaildelivery-managing unit 33 makes the time interval for retransmission ofemail arrival notice longer according to how large the counter is,thereby, reducing the increase of data traffic containing a lot ofuntransmittable data.

[0085] However, according to FIG. 5, when the counter is larger than acertain value, the time interval for retransmission of an email arrivalnotice is set to converge to a predetermined maximum time interval T.The object of this convergence is to meet the demand for quick emailtransmission, which is generally expected in an email delivery service,even when the counter has become quite large. It can be freelydetermined, on the basis of statistical data, and past experiences etc.,what the maximum limit should be for time interval T.

[0086] As a matter of course, the time interval for retransmission ofemail arrival notice, when gateway server 30 does not receive an Ack orgateway server 30 receives a NAck, can be determined on the basis ofother relations than the one illustrated in FIG. 5.

[0087] (2) In the Instance that a Busy Ack is Received

[0088] When gateway server 30 receives a busy Ack, as mentioned above,gateway server 30 uses a certain predetermined time interval as the timeinterval for the retransmission of an email arrival notice.

[0089] In this instance, mobile station 10 is inside the area where itcan communicate, but mobile station 10 cannot return to the state ofbeing able to communicate till the current communication ends.Therefore, it is preferable that the time interval should be determinedaccording to the average time required for ordinal communication.

[0090] (3) In the Instance that Email Delivery Fails

[0091] In the instance that gateway server 30 receives a receptionfailure notice from mobile station 10 through mobile packetcommunication network 20, as mentioned above, gateway server 30 uses acertain predetermined time interval as the time interval for aretransmission of the email arrival notice.

[0092] In the instance that mobile station 10 fails to receive an emailbecause it has been in the state of not being able to communicate for ashort time, there is a strong possibility that the reason why mobilestation 10 failed to receive an email is a temporary one, such as thedecrease in the strength of the radio wave received by mobile station 10because of the presence of surrounding buildings, etc. Due to thetemporary nature of the communication status of reception failure, itcan be assumed that there is a strong possibility that mobile station 10will return to the status of being able to communicate after a shorttime period. Therefore, it is preferable that the time interval forretransmission of email to mobile station 10 be determined to be a shorttime period.

[0093] (4) Comparison Among Time Intervals for Retransmission of EmailArrival Notice

[0094] Because of the reasons mentioned above, generally speaking, it ispreferable that time intervals to instances (1), (2), and (3) aredifferent from each other in the relations mentioned below.

[0095] The time interval in the instance that a delivery failure noticeis received is smaller than the time interval in the instance that abusy Ack is received, and the time interval in the instance that a busyAck is received is smaller than the time interval in the instance thatno Ack is received or a NAck is received.

[0096] [1.4] Modifications

[0097] (1) Use of Received Radio Wave Strength Information of MobileStations for Determining Email Arrival Notice Retransmission Times

[0098] In the embodiment explained above, gateway server 30 evaluatesthe current status of communication of mobile station 10 on the basis ofan Ack, or a delivery failure notice, received through mobile packetcommunication network 20, and determines the time when it retransmits anemail arrival notice according to the communication status of mobilestation 10. In addition to the information on the status ofcommunication, information of received radio wave strength of mobilestation 10 can be also used for determining the times when email arrivalnotice is retransmitted and when email is transmitted. In such a case,the following configuration needs to be adopted.

[0099]FIG. 7 is a diagram showing a configuration of mobile station 10in this modification. In FIG. 7, the components, which are the same asthose in FIG. 2 are provided with the same symbols as in FIG. 2. Asshown in FIG. 7, mobile station 10 in this embodiment comprises receivedradio wave strength-measuring unit 15 in addition to all the componentsof mobile station 10 shown in FIG. 2. When transmitting and receiving,unit 12 receives beacon signals transmitted by base station 21, andreceived radio wave strength-measuring unit 15 measures the strength ofthe received radio wave of the beacon signals.

[0100] In this modification, when CPU 111 of mobile station 10 receivesan email arrival notice when mobile station 10 is able to communicate,it transmits an Ack with information regarding the strength of thereceived radio wave, which is measured by received radio wavestrength-measuring unit 15. The information regarding the strength ofthe received radio wave, which is attached to the Ack, can be expressedin any kind of format. For example, CPU 111 can prepare received radiowave strength indexes, which are classified into 5 stages, namelyintegral numbers from ‘1’ to ‘5’, according to the measured values ofthe radio wave strength, and transmit the Ack along with the receivedradio wave strength indexes. The correspondence between the receivedradio wave strength indexes and the strength of the received radio wavecan be defined freely according to the situation, and it is usuallydefined on the basis of statistical data or past experiences.

[0101] In a system where the information of the strength of the receivedradio wave is used, for example, email delivery-managing unit 33 ofgateway server 30 reads out the information of the strength of thereceived radio wave, which is attached to a normal Ack, before ittransmits the email corresponding to the normal Ack. The email isactually transmitted only when the strength of the received radio waveof mobile station 10 is greater than a predetermined threshold value.According to this system, the transmission of emails can be performedwith greater certainty, and a traffic congestion due to an accumulationof data from failed email transmission, can be reduced.

[0102] In the above-mentioned modification, mobile station 10 transmitsan Ack with information of the strength of the received radio wave, andemail delivery-managing unit 33 of gateway server 30 determines whetherto retransmit an email arrival notice and transmits an email on thebasis of the information attached to the Ack received from mobilestation 10. The present invention is not limited to the above describedfunction, and it is also possible that CPU 111 creates informationregarding the transition of strength of the received radio wave usingthe information of the strength of the received radio wave of mobilestation 10, which is measured by the received radio wavestrength-measuring unit 15, and CPU 111 transmits the Ack with theinformation regarding the transition. In this case, emaildelivery-managing unit 31 of gateway server 30 determines whether toretransmit an email arrival notice and transmits an email on the basisof the information regarding the transition of strength of the receivedradio wave.

[0103] In a system where the information regarding the transition ofstrength of the received radio wave is used, email delivery-managingunit 33 of gateway server 30 reads out the information regarding thetransition of strength of the received radio wave attached to NAck whenit receives a Nack; and evaluates the present status of mobile station10, as being under the status of ‘decrease of radio wave strength causedby roaming’, on the basis of the information, and determines the retrialtime for the transmission of email arrival notice. According to thissystem, the deterioration of the speed of email delivery can becontrolled, and also the increase of data traffic caused by failed emailarrival notice transmissions can be reduced when mobile station 10 isnot able to communicate.

[0104] (2) Arrangement of Components of Email Sever System

[0105] In the embodiment explained above, the components of a system ofthe present invention, such as the means for receiving emails, the meansfor obtaining communication status of mobile stations, the means fordetermining transmissions of emails, and the means for instructingretrials of obtaining the communication status of mobile stations, areall installed in gateway server 30. However, all or some of thesecomponents can also be installed in several separated cabinets insteadof being installed together in the gateway server. In such a system, thecomponent groups, which are components grouped by the cabinets, can beconnected with each other by information communication network so as toenable them to work together.

1. A method of controlling email delivery, said method comprising: areceiving step for receiving an email, which is addressed to a mobilestation; a status obtaining step for obtaining information oncommunication status of said mobile station from a mobile communicationnetwork to which said mobile station belongs; a transmission determiningstep for determining whether said email should be transmitted to saidmobile station at the present moment or not, on the basis of saidinformation on the communication status; a transmitting step fortransmitting said email if said email is determined to be transmitted insaid transmission determining step, and; a retrial instructing step forsetting a waiting time on the basis of said information on thecommunication status, and instructing the execution of obtainingoperation by said status obtaining step following said waiting time, ifsaid email is determined to not be transmitted in said transmissiondetermining step, or if said email is determined to be transmitted insaid transmission determining step and transmission of said email fails.2. A method of controlling email delivery according to claim 1, wherein:said method further comprises an arrival notice transmitting step fortransmitting a notice of arrival of said email to said mobile station,when said email, which is addressed to said mobile station, is received,and; information on communication status with regard to reception ofsaid notice of arrival is obtained as said information of communicationstatus in said status obtaining step.
 3. A method of controlling emaildelivery according to claim 1, wherein: said information on thecommunication status, which is obtained in said status obtaining step,indicates any of the following: communication possible status,communication busy status, or communication impossible status; in saidretrial instructing step, a first time period is set as said waitingtime if said information on the communication status indicatescommunication busy status, and a second time period, which is longerthan said first time period, is set as said waiting time if saidinformation on the communication status indicates communicationimpossible status.
 4. A method of controlling email delivery accordingto claim 1, wherein: said waiting time is determined on the basis ofnumber of times of execution of said status obtaining step for saidemail up to the present moment.
 5. A method of controlling emaildelivery according to claim 4, wherein: in said retrial instructingstep, if said number of times of execution of said status obtaining stepfor said email up to the present moment is the same as or more than apredetermined number, a predetermined time period is set as said waitingtime.
 6. A method of controlling email delivery according to claim 1,wherein: said information on the communication status is information ofstrength of received radio wave of said mobile station.
 7. A method ofcontrolling email delivery according to claim 1, wherein: saidinformation on the communication status is information of transition ofstrength of received radio wave of said mobile station.
 8. An emailserver, said email server comprising: a receiving means for receiving anemail, which is addressed to a mobile station; a status obtaining meansfor obtaining information on communication status of said mobile stationfrom a mobile communication network, to which said mobile stationbelongs; a transmission determining means for determining whether saidemail should be transmitted to said mobile station at the present momentor not on the basis of said information on the communication status; atransmitting means for transmitting said email if said email isdetermined to be transmitted by said transmission determining means,and; a retrial instructing means for setting a waiting time on the basisof said information on the communication status, and instructing theoperation of said status obtaining means following said waiting time, ifsaid email is determined not to be transmitted by said transmissiondetermining means.